Comments on Potentially Hazardous Asteroid 1999 AN10

Asteroid 1999 AN10 made the news recently because, according to
a group of researchers in Italy, there is a remote possibility
that it could collide with the Earth in the year 2039. Writing
in a scientific paper submitted for publication, researchers
Andrea Milani, Steven R. Chesley and Giovanni B. Valsecchi say that
the chance of a collision in 2039 is exceedingly small, only about
one in a billion, but they add that the asteroid's orbit will remain
threateningly close to the Earth's orbit for many centuries to come.

Although the threat posed by 1999 AN10 must certainly be taken
seriously, the probability of impact for this object is so
miniscule that the authors of the paper felt no great urgency to
inform the press of the new calculations, and the other NEO
scientists reviewing the paper agreed with this policy.
To put it into perspective, consider that the probability of
1999 AN10 impacting in 2039 is tens of thousands of times less
than the probability of an undiscovered asteroid of equivalent size
hitting the Earth during the same 40-year period. Furthermore, in
just a few months, 1999 AN10 will be observed again, as it moves
back into the nighttime sky, and the new data will, in all
likelihood, completely eliminate the possibility of impact in 2039.
Researchers should then be able to start examining the possibility
of impacts after 2039.

As it turned out, the Milani et al. paper was publicized not by the
authors, but by a third party who found it accidentally on one of
the author's web pages; the authors were not even consulted before
their results were publicized. An internet debate ensued on such
issues as why the results had not been made public, and whether or
not the paper had been peer-reviewed to ensure accuracy. The
reasons for not making the results public have already been
described: basically, there was no great urgency to publicize a
one-in-a-billion-chance impact 40 years from now, when even that
remote a possibility will likely disappear in a few months.

On the issue of peer review, Milani and his colleagues followed a
commendable course. The authors distributed their paper to
qualified experts more than a week before placing the paper on
their web page, seeking confirmation of their results. Our group
at JPL examined the paper and saw no major flaws. We have also
confirmed the existence of the impact scenario for 2039, and we
confirm that the probability of impact in 2039 is about one in a
billion.